1. Field of the Invention
This invention relates to a system for controlling the operation of a semiconductor laser, and, in particular, to a system for controlling the operation for turning on or turning off of a semiconductor laser. More specifically, the present invention relates to a system for controlling the operation of a semiconductor laser for use in a light scanning system, such as a laser printer, as a source for emitting a laser beam.
2. Description of the Prior Art
A semiconductor laser is being widely used as a source for emitting a laser beam, for example, in a light scanning system, such as a laser printer. When used as a beam source in a laser printer, a modulation signal carrying image information must be supplied for driving a semiconductor laser which then emits a modulated laser beam carrying image information. Such a modulated laser beam is then passed through an optical system including a rotating beam deflector, such as a polygonal mirror, and focusing means, such as f.theta. lens, to be focused onto an imaging surface, such as a photosensitive surface, in a scanned fashion, so that an electrostatic latent image is formed on the imaging surface in accordance with the image information.
A typical prior art semiconductor laser modulation system is shown in FIG. 1, and it includes a constant current source comprised of a constant voltage source 1, a PNP transistor 2, and a resistor 3, which supplies a constant current to a semiconductor laser 11 through a differential switching circuit comprised of a pair of PNP transistors 4 and 5 and resistors 6 through 10. In this system, an image-modulated signal is applied to an input terminal 12, and the differential amplifier controls the on/off operation of the semiconductor laser 11 in accordance with the image-modulated signal. Such a prior art system for controlling the operation of the semiconductor laser calls for the provision of a differential amplifier and thus it is rather complicated in structure and expensive to manufacture.
On the other hand, it is known in the art that the semiconductor laser is very sensitive to the surrounding temperature, and the intensity of the laser output fluctuates significantly depending on the temperature of the surrounding atmosphere. Thus, it is often required to provide means for stabilizing the laser output in the semiconductor laser operating system. One known viable technique for stabilizing the output of a semiconductor laser is to use a counter for controlling the laser power output in a feedback loop. In this prior art system, the feedback loop includes a monitoring photodetector for detecting the current intensity of the laser beam emitted from a semiconductor laser and its detected intensity is compared with a predetermined reference level, whereby the count of the counter is updated so as to make the difference between the detected and reference values zero and the updated count is used to suitable vary the level of driver current to be supplied to the semiconductor laser. In such a prior art semiconductor laser controlling system, there is a danger that an undesirably large driving current could be supplied to the semiconductor laser, thereby destroying the laser or causing the laser to malfunction, and, thus, there is a need to provide some kind of a safety feature so as to avoid such an undesired situation to happen.